Most camper chemicals are "Odor Masks" that work on small tanks that are frequently dumped. Many/most actually kill the good bacteria that is needed to prevent the sulfide gasses from being formed.
Odorlos is a nitrate containing product that promotes bacterial action in the tank to mitigate and reduce the sulfide gasses.
Below is a wordy article that you might be interested in.
http://www.rvlifestyleexperts.com/fu...AND_BEYOND.pdf
Suggest you give your tank the best cleaning it ever had and start fresh with KO or Oderlos.
Come back in a month and tell us your results. Can I suggest a bigger tank or more frequent pump outs.
Another thing to look at is the amount of flush water you use with each flush. Is there a way to regulate it?